Functional preserving carbon dots-based fluorescent probe for mercury (II) ions sensing in herbal medicines via coordination and electron transfer

Simultaneous detection of multiple microRNAs based on fluorescence resonance energy transfer under a single excitation wavelength

MicroRNAs (miRNAs) play a key role in physiological processes, and their dysregulation is closely related to various human diseases. Simultaneous detection of multiple miRNAs is pivotal to cancer diagnosis at an early stage. However, most multicomponent analyses generally involve multiple excitation wavelengths, which are complicated and often challenging to simultaneously acquire multiple detection signals. In this study, a convenient and sensitive sensor was developed to simultaneously detection of multiple miRNAs under a single excitation wavelength through the fluorescence resonance energy transfer between the carbon dots (CDs)/quantum dots (QDs) and graphene oxide (GO). A hybridization chain reaction (HCR) was triggered by miRNA-141 and miRNA-21, resulting in the high sensitivity with a limit of detection (LOD) of 50 pM (3σ/k) for miRNA-141 and 60 pM (3σ/k) for miRNA-21. This simultaneous assay also showed excellent specificity discrimination against the mismatch. Furthermore, our proposed method successfully detected miRNA-21 and miRNA-141 in human serum samples at a same time, indicating its diagnostic potential in a clinical setting.

Transforming bio-waste lignin into amine functionalized carbon quantum dots for selective detection of trace Cu2+ in aqueous system

Developing carbon quantum dots (CQDs) from bio-waste lignin for effectively detecting Cu2+ is of great significance for promoting the value-added utilization of lignin resources. However, the limited amount of surface-active groups and low quantum yield of lignin-based CQDs hinder their application in this regard. Herein, bio-waste lignin was converted into value-added amine functionalized CQDs using a facile two-step hydrothermal approach. The as-synthesized CQDs modified with amino groups exhibit bright green fluorescence, abundant surface functional groups, high water solubility and uniform particle size (3.9 nm). Systematic analysis demonstrates that the rich NH2 groups (~12.3 %) on the CQDs backbone improve their fluorescence properties (quantum yield increased from 3.4 % to 21.1 %) and specific detection ability for Cu2+. The developed NH2-CQDs serve as an efficient fluorescent probe, displaying high sensitivity and selectivity towards Cu2+ in aqueous system, with a detection limit of 2.42 μmol/L, which is lower than the maximum permitted amount of Cu2+ in drinking water (20 μmol/L). The detection mechanism of NH2-CQDs for Cu2+ is attributed to the synergy of static quenching and photo-induced electron transfer. This study provides a valuable reference for the synthesis of high-quality fluorescent CQDs from lignin resources and the effective detection of trace Cu2+ in aquatic environments.

A photoluminescent and electrochemiluminescent probe based on an iridium(III) complex with a boronic acid-functionalised ancillary ligand for the selective detection of mercury(II) ions

Exposure to mercury(II) ions (Hg2+) can cause various diseases such as Minamata disease, acrodynia, Alzheimer's disease, and Hunter-Russell syndrome, and even organ damage. Therefore, real-time and accurate monitoring of Hg2+ in environmental samples is crucial. In this study, we report a photoluminescent (PL) and electrochemiluminescent (ECL) probe based on a cyclometalated Ir(III) complex for the selective detection of Hg2+. The introduction of a reaction site, o-aminomethylphenylboronic acid, on the ancillary ligands allowed a prompt transmetalation reaction to take place between Hg2+ and boronic acid. This reaction resulted in significant decreases of the PL and ECL signals due to the photo-induced electron transfer from the Ir(III) complex to the Hg2+ ions. The probe was applied to the selective detection of Hg2+, and the signal changes revealed a linear correlation with Hg2+ concentrations in the range of 0-10 μM (LOD = 0.72 μM for PL, 8.03 nM for ECL). The designed probe allowed the successful quantification of Hg2+ in tap water samples, which proves its potential for the selective detection of Hg2+ in environmental samples.

Carbon dots with aggregation-induced emission enhancement (AIEE) for detection of Zr4+/ Hf4+ and PTP1B activity

The frequent use of Zirconium (Zr) and Hafnium (Hf) in modern industries may result in serious environment issues, and thus developing analytical methods to facilitate the control of these two resembled metal elements is urgently needed. However, up to now, rapidly and conveniently detecting Zr⁴⁺ and Hf⁴⁺ is still full of challenge. In this study, nitrogen and sulfur co-doped carbon dots (N, S-Cdots) with aggregation-induced emission enhancement (AIEE) were prepared and used for turn-on detection of Zr⁴⁺ and Hf⁴⁺ within 10 min. The photoluminescent intensity of N, S-Cdots showed a good linear correlation with Zr⁴⁺ and Hf⁴⁺concentrations ranging from 0 to 30 μM with the limit of detection (LOD) of 0.47 and 0.53 μM, respectively. Furthermore, this fluorometric assay was successfully used for quantitative analysis of Zr⁴⁺ and Hf⁴⁺ in real water samples with satisfactory recoveries in the range of 91.16-108.50% and 90.28-106.49%, respectively. Finally, the developed N, S-Cdots sensing system was used for assay PTP1B activity and screening its inhibitor with Zr⁴⁺ as the medium. Our work demonstrated that the as-prepared N, S-Cdots with AIEE can offer a simple and reliable alternative for rapid detection of Zr⁴⁺ and Hf⁴⁺ in water samples, in addition to being potential useful in phosphatase analysis and drug discovery.

Sequential Injection Analysis for Rapid Determination of Mercury in Skincare Products Based on Fluorescence Quenching of Eco-Friendly Synthesized Carbon Dots

This work reports the analysis of mercury using a spectrofluorometric method combined with a sequential injection analysis (SIA) system. This method is based on the measurement of fluorescence intensity of carbon dots (CDs), which is quenched proportionally after adding mercury ions. Herein, the CDs underwent environmentally friendly synthesis using a microwave-assisted approach that provides intensive and efficient energy and shortens reaction time. After irradiation at 750 W for 5 min in a microwave oven, a dark brown CD solution with a concentration of 2.7 mg mL^-1 was obtained. The properties of the CDs were characterized by transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and UV-vis spectrometry. We presented for the first time the use of CDs as a specific reagent for the determination of mercury in skincare products with the SIA system to achieve rapid analysis and full automatic control. The as-prepared CD stock solution was diluted 10 times and used as a reagent in the SIA system. Excitation and emission wavelengths at 360 and 452 nm, respectively, were used to construct a calibration curve. Physical parameters affecting the SIA performance were optimized. In addition, the effect of pH and other ions was investigated. Under the optimum conditions, our method showed a linear range from 0.3 to 600 mg L^-1 with an R ² of 0.99. The limit of detection was 0.1 mg L^-1. Relative standard deviation was 1.53% (n = 12) with a high sample throughput of 20 samples per hour. Finally, the accuracy of our method was validated by comparison using inductively coupled plasma mass spectrometry. Acceptable recoveries were also presented without a significant matrix effect. This method was also the first time that uses the untreated CDs for the determination of mercury(II) in skincare products. Therefore, this method could be an alternative for mercuric toxic control in other sample applications.

Machine Learning System To Monitor Hg2+ and Sulfide Using a Polychromatic Fluorescence-Colorimetric Paper Sensor

An optical monitoring device combining a smartphone with a polychromatic ratiometric fluorescence-colorimetric paper sensor was developed to detect Hg2+ and S2- in water and seafood. This monitoring included the detection of food deterioration and was made possible by processing the sensing data with a machine learning algorithm. The polychromatic fluorescence sensor was composed of blue fluorescent carbon quantum dots (CDs) (BU-CDs) and green and red fluorescent CdZnTe quantum dots (QDs) (named GN-QDs and RD-QDs, respectively). The experimental results and density functional theory (DFT) prove that the incorporation of Zn can improve the stability and quantum yield of CdZnTe QDs. According to the dynamic and static quenching mechanisms, GN-QDs and RD-QDs were quenched by Hg2+ and sulfide, respectively, but BU-CDs were not sensitive to them. The system colors change from green to red to blue as the concentration of the two detectors rises, and the limits of detection (LOD) were 0.002 and 1.488 μM, respectively. Meanwhile, the probe was combined with the hydrogel to construct a visual sensing intelligent test strip, which realized the monitoring of food freshness. In addition, a smartphone device assisted by multiple machine learning methods was used to text Hg2+ and sulfide in real samples. It can be concluded that the fabulous stability, sensitivity, and practicality exhibited by this sensing mechanism give it unlimited potential for assessing the contents of toxic and hazardous substances Hg2+ and sulfide.

The safety of Chinese medicine: A systematic review of endogenous substances and exogenous residues

BACKGROUND

Safety and toxicity have become major challenges in the internationalization of Chinese medicine. Inspite of its wide application, security problems of Chinese medicine still occur from time to time, raising widespread concerns about its safety. Most of the studies either only partially discussed the intrinsic toxicities or extrinsic harmful residues in Chinese medicine, or briefly described detoxification and attenuation methods. It is necessary to systematically discuss Chinese medicine's extrinsic and intrinsic toxic components and corresponding toxicity detoxification or detection methods as a whole.

PURPOSE

This review comprehensively summarizes various toxic components in Chinese medicine from intrinsic and extrinsic. Then the corresponding methods for detoxification or detection of toxicity are highlighted. It is expected to provide a reference for safeguards for developing and using Chinese medicine.

METHODS

A literature search was conducted in the databases, including PubMed, Web of Science,Wan-fang database, and the China National Knowledge Infrastructure (CNKI). Keywords used were safety, toxicity, intrinsic toxicities, extrinsic harmful residues, alkaloids, terpene and macrolides, saponins, toxic proteins, toxic crystals, minerals, heavy metals, pesticides, mycotoxins, sulfur dioxide, detoxification, detection, processing (Paozhi), compatibility (Peiwu), Chinese medicine, etc., and combinations of these keywords. All selected articles were from 2006 to 2022, and each was assessed critically for our exclusion criteria. Studies describe the classification of toxic components of Chinese medicine, the toxic effects and mechanisms of Chinese medicine, and the corresponding methods for detoxification or detection of toxicity.

RESULTS

The toxic components of Chinese medicines can be classified as intrinsic toxicities and extrinsic harmful residues. Firstly, we summarized the intrinsic toxicities of Chinese medicine, the adverse effects and toxicity mechanisms caused by these components. Next, we focused on the detoxification or attenuation methods for intrinsic toxicities of Chinese medicine. The other main part discussed the latest progress in analytical strategies for exogenous hazardous substances, including heavy metals, pesticides, and mycotoxins. Beyond reviewing mainstream instrumental methods, we also introduced the emerging biochip, biosensor and immuno-based techniques.

CONCLUSION

In this review, we provide an overall assessment of the recent progress in endogenous toxins and exogenous hazardous substances concerning Chinese medicine, which is expected to render deeper insights into the safety of Chinese medicine.

Inner-filter Effect Induced Fluorescence Quenching of Carbon Dots for Cr(VI) Detection with High Sensitivity

Carbon dots (CDs) were used to develop a sensitive sensing technique for detecting Cr(VI). CDs were made using a hydrothermal technique from citric acid and glutamic acid. These prepared CDs emitted blue fluorescence under excitation of 350 nm (λ_em = 420 nm), and the fluorescence quantum yield was 48.41%. Transmission electron microscope was used to examine the morphology of the CDs, which had an average size of 2.21 ± 0.39 nm. The elementary composition and bonding structure of the CDs were conducted by XPS and FT-IR spectrum. Cr(VI) quenched the fluorescence of CDs through a static quenching effect and an inner filter effect, allowing Cr(VI) to be detected quantitatively. This approach was used to detect Cr(VI) in two samples of water, with the findings demonstrating that it is reliable and accurate. The fluorescence intensity change was linearly related to the concentration of Cr(VI) in the range from 0.5 to 400 μM, with the detection limit being 0.10 μM. This approach has the virtues of wide detection range, low cost and fast response. The strategy has a great application prospect for detecting Cr(VI) in practical samples.

Simple synthesis of green luminescent N-doped carbon dots for malachite green determination

In this paper, novel N-doped carbon dots (N-CDs) were prepared from fuchsin basic and ethylenediamine tetraacetic acid-disodium salt (EDTA-2Na). The N-CDs were characterized by a series of techniques and it was found that the average particle size was 2.75 nm, and the surface had functional groups such as -NH₂ and -COOH. Interestingly, N-CDs exhibited a fast and sensitive response to malachite green (MG), which may be due to the inner filter effect (IFE). A method for the detection of MG in water samples from Jinyang Lake was developed using N-CDs, with a limit of detection (LOD) as low as 27.28 nM. Furthermore, N-CDs were utilized in the biological imaging of Arabidopsis thaliana.

Quantification of 2-chlorohydroquinone based on interaction between N-doped carbon quantum dots probe and photolysis products in fluorescence system

As a member of chlorophenolic compounds, 2-chlorohydroquinone (H₂QCl) has been widely used as intermediates in various chemical industries and leaded to serious threat on the environment. It is urgent to develop simple and robust analytical method for sensitive and selective determination of H₂QCl. Carbon quantum dots (CQDs), a promising photoluminescence nanomaterial, have gained sufficient concern as optical sensors owing to their outstanding photochemical properties. In this work, nitrogen doped carbon quantum dots (N-CQDs) were successfully synthesized by a simple secondary hydrothermal method and applied as a fluorescent probe for the quantitation of H₂QCl. A new fluorescence region centered at excitation wavelength of 310 nm and emission wavelength of 390 nm appeared after nitrogen doping. It was found that the N-CQDs exhibited a high selectivity towards H₂QCl with sensitive fluorescence response and the fluorescence quenching of N-CQDs was linear with the concentration of H₂QCl in the range of 30-90 μM (Y = 0.0049X + 0.1255, R² = 0.996). This is the first time that the dual role of excitation light was observed in the fluorescence detection system. The ultraviolet light acted as not only the excitation energy source for N-CQDs photoluminescence, but also the light source for photolysis of H₂QCl. In the detection process, H₂QCl was degraded to p-benzoquinone by light, and then the CQDs combined with p-benzoquinone through Michael addition reaction under the action of doped nitrogen. The electron transfer from N-CQDs to the linked p-benzoquinone caused the quenching of fluorescence originated from the edge state of N-CQDs. Furthermore, this established method can be applied for the quantitative determination of H₂QCl in environmental water samples with satisfactory recoveries between 94.31 and 105.51%.

Ultrasensitive Detection of Aqueous Uranyl Based on Uranyl-Triggered Protein Photocleavage

The detection of environmental uranyl is attracting increasing attention. However, the available detection strategies mainly depend on the selective recognition of uranyl, which is subject to severe interference by coexisting metal ions. Herein, based on the unique uranyl-triggered photocleavage property, the protein BSA is labelled with fluorescent molecules that exhibit an aggregation-induced emission effect for uranyl detection. Uranyl-triggered photocleavage causes the separation of the fluorescent-molecule-labelled protein fragments, leading to attenuation of the emission fluorescence, which is used as a signal for uranyl detection. This detection strategy shows high selectivity for uranyl and an ultralow detection limit of 24 pM with a broad detection range covering five orders of magnitude. The detection method also shows high reliability and stability, making it a promising technique for practical applications in diverse environments.

Ratiometric fluorescent nanoprobes based on carbon dots and multicolor CdTe quantum dots for multiplexed determination of heavy metal ions

Owing to the high risk to human and environmental health, heavy metal pollution has become a global problem. Rapid, accurate and multiplexed determination of heavy metal ions is critical. In this work, we reported a promising approach to designing ratiometric fluorescent nanoprobes for multiplexed determination of Hg²⁺, Cu²⁺, and Ag⁺ ions. The nanoprobes (CDs-QDx) were designed by mixing the CDs and multicolor CdTe QDs without the involvement of recognition elements. The CDs were insensitive to heavy metal ions while CdTe QDs showed the size-dependent fluorescence response to different heavy metal ions, thereby establishing a ratiometric detection scheme by measuring the fluorescence intensity ratios of CDs-QDx systems. By evaluating the detection performance, the CDs-QDx (x = 570, 650, and 702) were successfully used for differentiation and quantification of Hg²⁺, Cu²⁺, and Ag⁺ ions. In addition, we also carried out the detection of heavy metal ions in actual samples with acceptable results. We believed that this work offers new insight into the design of ratiometric fluorescent nanoprobe for multiplexed determination of not only heavy metals but also some other analytes by combining the CDs with CdTe QDs with fine-tuned sizes.

Highly fluorescent N doped C-dots as sensor for selective detection of Hg2+ in beverages

In this work, nitrogen doped carbon dots (NCDs) were synthesized through one step hydrothermal reaction using citric acid and ethylenediamine as precursors. The prepared NCDs exhibit high quantum yield of 67.4%, good stability, excellent selectivity and sensitivity. It was found that the NCDs have potential as a fluorescence sensor for the detection of Hg²⁺. Under optimal conditions, good linearity between the change in NCDs fluorescence intensity and Hg²⁺ concentration was obtained in the range of 0.3 to 2.0 μM with a detection limit at 0.24 μM. The possible detection mechanism was static quenching of NCDs by Hg²⁺. The established method was successfully applied to the determination of Hg²⁺ in beverage samples. The results indicated that the NCDs-based sensor has potential for detection of Hg²⁺ in real beverage sample.

Dual-Emitting Carbonized Polymer Dots Synthesized at Room Temperature for Ratiometric Fluorescence Sensing of Vitamin B12

Ratiometric fluorescence (FL) probes are highly desirable for highly sensitive and reliable assays. Dual-emitting carbonized polymer dots (CPDs) have great application prospects in building ratiometric FL sensors. However, dual-emitting CPDs are usually synthesized at high temperatures and high pressures, which not only increases the cost but also complicates the structure of CPDs. Here, we developed a facile strategy for the fabrication of dual-emitting CPDs at room temperature using tetrachlorobenzoquinone and ethylenediamine. The formation of CPDs was induced by Schiff base condensation reaction, enabling the following cross-linking polymerization process. The dual-emitting CPDs demonstrate good photostability and antioxidant capacity. Importantly, the typical dual-emission bands of the as-prepared CPDs are found to have a blue emission band at 445 nm with a maximum excitation of 350 nm and a yellow emission band at 575 nm with a maximum excitation of 440 nm. Based on the dual-emitting property of CPDs, a ratiometric FL nanoprobe is obtained for sensitive determination of vitamin B12 (VB12), as the inner filtering and static quenching effects between VB12 and CPDs allow effective quenching of the blue FL of CPDs, while the yellow FL is maintained. The established assay shows linear detection ranges of 0.25-100 μM with a low limit of detection of 0.14 μM. These findings provide new guidance for the facile preparation of CPDs with excellent dual-emitting optical properties, indicating good prospects in biosensing.

P-Doped Carbon Quantum Dots with Antibacterial Activity

It is a major challenge to effectively inhibit microbial pathogens in the treatment of infectious diseases. Research on the application of nanomaterials as antibacterial agents has evidenced their great potential for the remedy of infectious disease. Among these nanomaterials, carbon quantum dots (CQDs) have attracted much attention owing to their unique optical properties and high biosafety. In this work, P-doped CQDs were prepared by simple hydrothermal treatment of m-aminophenol and phosphoric acid with fluorescence emission at 501 nm when excited at 429 nm. The P-doped CQDs showed effective antibacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The minimal inhibitory concentrations (MICs) of P-doped CQD were 1.23 mg/mL for E. coli and 1.44 mg/mL for S. aureus. Furthermore, the morphologies of E. coli cells were damaged and S. aureus became irregular when treated with the P-doped CQDs. The results of zeta potential analysis demonstrated that the P-doped CQDs inhibit antibacterial activity and destroy the structure of bacteria by electronic interaction. In combination, the results of this study indicate that the as-prepared P-doped CQDs can be a promising candidate for the treatment of bacterial infections.

Use of nitrogen-doped amorphous carbon nanodots (N-CNDs) as a fluorometric paper-based sensor: a new approach for sensitive determination of lead(ii) at a trace level in highly ionic matrices

This work reports a facile synthesis of nitrogen-doped amorphous carbon nanodots (N-CNDs) and their use as a fluorometric paper-based sensor for the determination of Pb²⁺ at a low concentration. Both solution-based and paper-based systems were developed. The results show that the linearity ranges for Pb²⁺ determination were 0.010-10 mg L^-1 (LOD = 0.008 mg L^-1) and 0.005-0.075 mg L^-1 (LOD = 0.004 mg L^-1) for the solution-based and the paper-based sensors, respectively. Furthermore, the developed sensors show relatively high selectivity toward Pb²⁺ over ten other metal cations of different charges including As³⁺, Hg²⁺, Cd²⁺, Mg²⁺, Ni²⁺, Zn²⁺, Fe³⁺, Cu²⁺, Ba²⁺, and Ag⁺. The mechanism of Pb²⁺ determination was also investigated. It was found that the sensors exploited the quenching of the fluorescence intensity of N-CNDs by Pb²⁺via the photo-induced electron transfer (PET) mechanism. When applied to real water and herbal medicine samples, the performance of the sensor exhibited no significant difference as compared to the results of the validation method (ICP-OES). Overall, the developed sensors, especially the paper-based one, are promising for the practical analysis of Pb²⁺ in pharmaceutical and environmental samples with a low Pb²⁺ concentration.

Fluorescent wood with non-cytotoxicity for effective adsorption and sensitive detection of heavy metals

Heavy metal pollution is one of the primary challenges of water pollution, and the fabrication of highly effective, green and non-toxic adsorbents for heavy metals is urgently required on the basis of environmental and sustainable development strategies. Here, we report a novel fluorescent wood (FW) with effective adsorption ability (maximum theoretical adsorption capacity of 98.14 mg/g for hexavalent chromium [Cr(VI)]), good fluorescence properties (absolute quantum yield of 12.8%), non-cytotoxicity (cell viability of >90%) and high detection sensitivity and selectivity for Cr(VI). The FW was formed using a process involving delignification, infiltration with carbon dots, and free-radical polymerization with acrylic acid. Mechanistic analysis confirmed that the reconstructed 3D porous structure of the FW provided many effective sorption sites, such as amino, hydroxyl and carboxyl groups. This improved the adsorption ability and stabilized the fluorescence signal, which enhanced the detection ability. These factors give the novel FW considerable potential for use in the removal of Cr(VI) ions from wastewater.

An Active Surface Preservation Strategy for the Rational Development of Carbon Dots as pH-Responsive Fluorescent Nanosensors

Here we report the rational development of a carbon dot (CDs)-based fluorescent pH nanosensor by employing an active surface preservation strategy. More specifically, citric acid, urea and fluorescein were subjected to a one-pot hydrothermal treatment, which preserved fluorescein-like structures on the surface of the CDs. The obtained CDs showed pH-sensitive green emission, which can be used to determine pH variations from 3.7 to 12.1 by fluorescence enhancement. Moreover, the obtained nanoparticles showed excellent selectivity toward pH, fluorescence reversibility in different pH values, photostability, while being compatible with human cell lines (even at high concentrations). Furthermore, their performance as pH sensors was comparable with reference pH determination procedures. Thus, an active surface preservation strategy was successfully employed to develop fluorescence pH nanosensors in a rational manner and without post-synthesis functionalization strategies, which show potential for future use in pH determination.

Determination of Fe(Ⅲ) ion and cellular bioimaging based on a novel photoluminescent silicon nanoparticles

The determination approaches of Fe (Ⅲ) in biological samples were developed by a novel water-soluble silicon nanoparticles (SiNPs). The SiNPs were synthesized by a facile microwave-assisted method, and simultaneously featured strong blue fluorescence (photoluminescence quantum yield: 25.2%), long lifetime (~13.29 ns) and good photo-stability. The fluorescence intensities of SiNPs were gradually quenched with Fe (Ⅲ) concentration increasing from 2.0 to 50 μmol/L. The detection limit of the established method was 0.56 μmol/L and the precision for eleven replicate detections of 20 μmol/L Fe (Ⅲ) was 3.2% (relative standard deviation, RSD). The spiked recoveries were 99.0%-104.5%. Results of the lifetime decay and cyclic voltammetry (CV) evidenced that the electron transfer was responsible for the fluorescence quenching mechanism of SiNPs and Fe (Ⅲ). Moreover, the SiNPs were successfully applied in the determination of Fe(Ⅲ) in different environmental waters and human serum. Finally, the resulting SiNPs exhibited the green fluorescence in HeLa cells as the optical probe.

Surface chemistry tuning the selectivity of carbon nanodots towards Hg2+ recognition

Fluorescence quenching of carbon nanodots by metal ions has been extensively applied for the determination of oligonucleotides, proteins, small molecules and metal ions. However, the problem of poor selectivity originating from the coordination of surface oxygen-containing groups to many kinds of metal ions has limited the prosperity of carbon nanodots in detection field. Herein, the specific recognition of carbon nanodots to Hg²⁺ is controlled by rational regulation of the surface structure of carbon nanodots. Passivation of the surface carboxyl and hydroxyl groups plays a decisive role in inhibiting the binding of metal ions with carbon nanodots. Upon the attachment of Hg²⁺ specific recognition unit, carbon nanodots exhibited a high selectivity to Hg²⁺. This work facilitates to rationally design the surface structure of carbon nanodots to obtain the desirable selective recognition ability.

Carbon quantum dots derived from the extracellular polymeric substance of anaerobic ammonium oxidation granular sludge for detection of trace Mn(vii) and Cr(vi)

Carbon quantum dots (CQDs) were synthesized via a hydrothermal method, in which extracellular polymeric substance (EPS) from anaerobic ammonium oxidation (anammox) granular sludge was used as a carbon precursor, while citric acid and ethylenediamine were applied as auxiliary carbon source and passivation agent, respectively. The synthesized CQDs, with orderly spherical shape and mean size of 7.15 nm, emitted blue fluorescent light under UV radiation of 365 nm. The CQDs had a high fluorescence yield (40.84%), with good water solubility and excellent spectroscopic properties. In addition, the CQDs exhibited selective, sensitive and distinctive fluorescence quenching behaviors for Cr(vi) and Mn(vii) in a PBS buffer solution (NaH₂PO₄–Na₂HPO₄) of pH 7, with a detection limit of 5.8 nM for Cr(vi) and 2.3 nM for Mn(vii). Owing to the nitrogen components from the EPS of anammox granules, the CQDs were well nitrogen-doped, promoting electron-transfer and leading to reduction between the CQDs and Mn(vii)/Cr(vi). These results indicate that CQD-based chemical sensing is a simple and efficient means for the fluorescence detection of Mn(vii) and Cr(vi).

Fluorescence detection of trace Mn(vii) or Cr(vi) by the CQDs prepared from anaerobic ammonium oxidation granular sludge.

"On-off" ratiometric fluorescent detection of Hg2+ based on N-doped carbon dots-rhodamine B@TAPT-DHTA-COF

Covalent-organic frameworks (COFs) are new porous crystalline materials owning outstanding stability, adsorbability and hypotoxicity. The assembly of fluorescence probes into porous COF provides a good method for ratiometric fluorescence detection avoiding the toxic effects of fluorescence probes to the samples. Herein, a two-dimensional COF (TAPT-DHTA-COF) was employed as a host to encapsulate N-doped carbon dots (NCDs) and Rhodamine B (RhB) (NCDs-RhB@COF). NCDs and RhB were uniformly assembled into the pores of TAPT-DHTA- COF based on the hydrogen bond. The as-prepared NCDs-RhB@COF nanocomposites exhibited blue emission of NCDs at 440 nm and red emission of RhB at 570 nm at excitation of 340 nm. After the addition of Hg²⁺, the blue emission became weaker while the red emission was enhanced due to the strong coordination between NCDs-RhB@COF and Hg²⁺. This "on-off" fluorescence probe was applied in detection of trace Hg²⁺ with linear range of 0.048-10 μM and detection limit of 15.9 nM together with appropriate selectivity, acceptable sensitivity and stability. The work shreds some light for COF as platform to construct ratiometric fluorescent sensor for industrial and biological application.

Carbon dot-based composites for catalytic applications

We summarize the construction methods and influencing factors of CDs-based composites and discuss their catalytic applications, including photocatalysis, chemical catalysis, peroxidase-like catalysis, Fenton-like catalysis and electrocatalysis.

Acid Oxidation of Muskmelon Fruit for the Fabrication of Carbon Dots with Specific Emission Colors for Recognition of Hg2+ Ions and Cell Imaging

In this study, water-soluble emissive carbon dots (CDs) are effectively fabricated with specific optical properties and colors by acid oxidation of muskmelon (Cucumis melo) fruit, which are termed as C. melo CDs (CMCDs). The fluorescence properties of CMCDs were tuned by controlling the experimental conditions that allow them to emit different colors, that is, blue (B-), green (G-), and yellow (Y-) CMCDs, with different emission wavelengths at 432, 515, and 554 nm when excited at 342, 415, and 425 nm, respectively. The fabricated multicolor-emissive CDs were confirmed by various analytical techniques. The sizes of B-, G-, and Y-CMCDs were found to be ∼3.5, ∼4.3, and ∼5.8 nm, respectively. The as-prepared CMCDs display stable emissions with quantum yields of 7.07, 26.9, and 14.3% for the three CMCDs, which could act as a promising probe for the selective detection of Hg²⁺ ions. Upon the addition of Hg²⁺ ions, the fluorescence intensity of G-CMCDs at 515 nm was quenched largely than that of B- and Y-CMCDs. The spectroscopic results display that the G-CMCDs acted as a sensor for the detection of Hg²⁺ ions with a wide linear range from 1.0 to 25 μM (R ² = 0.9855) with a detection limit of 0.33 μM. This method was successfully applied to detect Hg²⁺ ions in biological and water samples. The fabricated multicolor-emissive CMCDs possess the cell (Cunninghamella elegans, Aspergillus flavus, and Rhizoctonia solani) imaging property, suggesting the biocompatible nature for multicolor imaging of various cells.

Carbon dots-based fluorescent turn off/on sensor for highly selective and sensitive detection of Hg2+ and biothiols

In this work, sodium salicylate and ethylenediamine (EDA) are used as the precursors to synthesize green fluorescent carbon dots (CDs). The CDs have some attractive properties, including better oxidation resistance, good water solubility, and excellent stability in high ionic strength solutions in a pH range of 6.0-10.0. Compared to other metal ions, only Hg²⁺ can quench the fluorescence of CDs, and with the introduction of biothiols, the fluorescence of the CDs/Hg²⁺ system can be recovered. Therefore, a turn off/on fluorescent sensor is constructed using CDs as a fluorescent probe, and the sensor is applied to the detection of Hg²⁺ and biothiols (glutathione, homocysteine and cysteine). In addition, the fluorescent sensor exhibits excellent selectivity and sensitivity. The linear range of Hg²⁺ is 0.05-10 μM with the detection limit of 44 nM. Glutathione, homocysteine, and cysteine have a linear response in the range of 0.5-10 μM with the limit of detection of 80, 76, and 69 nM, respectively. Furthermore, the fluorescence method is successfully used to detect Hg²⁺ in actual water samples and biothiols in human plasma.

N-doped carbon dots under Xenon lamp irradiation: Fluorescence red-shift and its potential mechanism

Despite extensive research on carbon dots (CDs), rare studies have been performed on the photostability of CDs. Here, the photostability of CDs synthesized with 3-aminobenzoic acid were systematically investigated under different pH conditions (5.0, 7.4 and 9.0). The results showed that under Xenon lamp irradiation, the fluorescence (FL) intensity of the CDs exhibited an increase first and then a decrease, with a gradual shift of the maximum emission wavelength to longer wavelength. Further investigation indicated that the irradiation induced the change of the CD surface functional groups and gave rise to aggregation, resulting in the formation of larger particles. This study provided important reference value towards research on CDs properties.